Steve Knight says...

been using my delta 14" saw for about 10 years. been using the same blades
lenox bi metal blades forever too.
the first time it happed I was removing a piece of wood I cut only part way.
sometimes the wood binds a bit and pulls the blade. but it does not cause a
problem. but now the blade will jump forward and be about 1/2 on and 1/2 off the
wheel. at first I played around with the tracking to get it normal. but I found
that if I just undo the tension and get the blade in the middle and tighten it
it will move again the first time and stay the second usually. till something
causes it to jump 1/2 off the wheel again.
it tracks fine till that sudden jump. then it will stay there untilled I change
the tracking or undo and redo the tension.
the second time it happened it cut right through the blade guard sure startled
me (G)

Just a few guesses that you probably already thought of. Maybe some
embedded saw dust on the tires or just worn tires. Maybe the tires are
concave after years of use. Maybe there is a loose spot where the glue
let go or maybe there is a crack in a tire. I would change the tires if
it keeps happening.